Because of its fast metabolism gadolinium as a commercial drug was not considered to be suitable for neutron capture therapy. We studied additive effect of gadolinium and boron co-administration using colony forming assay. As a result, the survival of tumor cells with additional 5ppm of Gd-DTPA decreased to 1/10 compared to the cells with boron only. Using gadolinium to increase the effect of BNCT instead of additional X-ray irradiation might be beneficial, as such combination complies with the short-time irradiation regimen at the accelerator-based neutron source. •Gd-DTPA is widely clinically used as a contrast medium for MRI.•Shift to an accelerator-based neutron source is advantageous for gadolinium NCT.•Boron–gadolinium NCT effects on tumor cell lines were significant.•Additional administration of Gd-DTPA might enhance the effect of BPA–BNCT.